1. Field of the Invention
The present invention generally relates to an implantable medical system including an implantable medical device and an implantable medical lead for pacing the HIS Bundle.
2. Description of the Prior Art
Many studies have recently demonstrated the potential detrimental effects of RV (Right Ventricular) apex pacing. The stimulation of the ventricles through RV apex generates an activation pattern similar to LBBB (Left Bundle Branch Block) patients, with unsynchronized right and left ventricles as a result. The long term effects of RV apical pacing are thought to be (in the worst case) advancing CHF (Cardiac Heart Failure) or compromised hemodynamics.
One way to overcome these disadvantages is to implant a CRT (Cardiac Rhythm Therapy) system with a LV (Left Ventricular) that can restore the synchronization between the left ventricle and the right ventricle. However, for those patients which have an intact conduction system below the AV-node, pacing in upper regions in the RVOT/RV septal wall—or more beneficial—the Bundle of HIS, may restore synchronization as well.
Pacing in HIS Bundle may actually generate EGM's being more or less identical with the intrinsic or native ones. The pacing signal is propagated along the Purkinje fiber system and the patient will benefit from having a true physiologic pacing. However, pacing of the HIS Bundle area is difficult to achieve and/or optimize. It is a small area located deep in the tissue between the RA (Right Atrium) and RV, near the membranous septum or the interventricular septum. In addition, the depth at which the intrinsic conduction system lies is individual to every person.
Due to the fact that the Bundle of HIS is a structure located deep within the myocardium, the positioning of the medical lead must be optimized in three dimensions. Typically, the Bundle of HIS is reached from the atrium by mapping in the area of the Triangle of Koch. The correct region at which the endocardium should be penetrated with the lead in order to reach the Bundle of HIS is identified by finding the spot where the largest HIS Bundle potential is measured. The lead helix is then fixated in this spot.
However, studies have revealed that both the capture threshold and the sense thresholds as well as the ability to distinguish the signal from the HIS Bundle with the implantable lead is to a high degree dependent on the depth of the electrode (i.e. the helix) in the tissue, see, for example, Deshmukh et al., Circulation 2000; 101; 869-877 and Laske et al., PACE 2006; 29; 397-405. With a conventional lead, which has a length of the helix of approximately 1.5-2.0 mm, the helix may in some cases never reach the optimal site. In order to reach the desired spot in close proximity to the bundle of HIS, a longer helix will be required. This has been studied in Karpawich et al., PACE, Part II, 1992; 15; 2011-2015, where a helix having a length of 4.5 mm was used to pace HIS Bundle. A long conductive helix is however associated with negative side effects. For example, the depolarization of the tissue may become indistinctive relative the Bundle of HIS and a large conductive surface area of the helix will entail a reduced impedance.
In U.S. Pat. No. 7,177,704, a medical lead including a helix for pacing HIS Bundle is shown. The helically shaped electrode is partially masked with an insulative material, leaving an intermediate area unmasked and electrically conductive to allow for pacing at specific depths within the heart tissue. During the implantation, the physician rotates the electrode in and out of the tissue to determine an optimal position in which to pace the portion of the intrinsic conductive system. Since the depth at which the intrinsic system lies is different for every person, the physician rotates the helical electrode in and out of the tissue to determine an optimal conductive position. Consequently, the implantation procedure of the medical lead according to U.S. Pat. No. 7,177,704 may be cumbersome and time-consuming.
Thus, there is still a need within the art for an improved implantable medical device and an implantable medical lead capable of providing a reliable and accurate pacing of the His Bundle. There is also a need within the art of an improved implantable medical device and an implantable medical lead for stimulation of the His Bundle that can be implanted in an efficient way.